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Abstract

Encryption is used to protect information in sys- tems where sensitive voice is transmitted over-the- air. The receiving radio must know the algorithm and key used in the encryption process by the transmitter.

Copyright

Motorola Inc. November 1996

Country

United States

Language

English (United States)

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Page 1 of 2

M-LA Technical Developments

PROPER CODE DlXECTOR UTILIZING DECODED PITCH

by Esteban Yepez, III and Ferdie Brillantes

1. INTRODUCTION

Encryption is used to protect information in sys- tems where sensitive voice is transmitted over-the- air. The receiving radio must know the algorithm and key used in the encryption process by the transmitter.

The APCO25 Common Air Interface (CAI) pro- vides an Algorithm Identifier (ALGID) and a Key Identifier (KID) to assist the receiver in selecting the proper algorithm and key.

The APC025 vocoder divides the audio into 20 ms frames. The IMBE vocoder calculates and encodes the hmdamental tiequency, or pitch, of each audio frame. A total of 18 encoded audio frames are con- tained in each Superframe along with the ALGID and KID mentioned previously.

If the receiver does not recognize either identi- fier, all 18 frames of the encrypted, encoded audio are discarded and the speaker is muted.

Radio Bn

Keys Disagree

Key Table r+

/

0:$54367

t!$234A5 2!$AC456 3:$1234

.

\

--7&=02

ALGID = 99

ALGID = 99

Matching KID and ALGID does not guarantee that the correct key will be used to decrypt the audio signal. The actual key associated with the KID in the transmitter may differ from the corresponding key in the receiver. Therefore, the parameters used by the decoder will remain corrupted and will result in the generation of invalid audio samples.

mine if the correct Key was used to decrypt the received signal. This technique measured the degree o...